Most of the increase in May was due to a big jump in volatile commercial aircraft demand. Still, businesses also ordered more machinery, computers and household appliances.

A category of orders that’s viewed as a proxy for business investment plans — which excludes the volatile areas of transportation and defense — rose 1.5 percent. That was even stronger than solid gains in the previous two months.

This measure of business investment hadn’t increased for three straight months since the fall of 2011. The consecutive gains suggest US manufacturing could improve in the second half of the year.

Manufacturing has struggled this year after helping propel the economy in the first three years after the recession. US factories have seen less demand for exports because of weaker global growth.

The gain in factory orders follows a report that shows manufacturing activity picked up in June. The Institute for Supply Management’s index of manufacturing activity rose to 50.9 from 49. Readings above 50 indicate expansion.